The structure of the proton has been determined in previous experiments and is known to consist of three main or "valence" quarks, confined within the proton by extremely strong forces. These forces can be equivalently regarded as being due to the exchange of force particles called gluons, which ocasionally transform into quark-antiquark-pairs. These pairs and the gluons themselves are present in high quantity in the proton. When an electron collides with a proton at HERA energy, the electron can strike either a valence quark or some of the other components of the proton. The investigation of these interactions using the higher resolving power of the HERA beam is the main aim of the experiment.
